Carbon nanotubes (CNTs) are fullerene-related structures of graphite cylinders and were first synthesized by Iijima (Iijima S, Helical microtubes of graphitic carbon, Nature 1991, 354, 56). Single walled nanotubes (SWNTs) consist of single layers of graphite lattice rolled into cylinders, whereas multiwalled nanotubes (MWNTs) consist of sets of concentric cylindrical shells, each of which resembles a SWNT. Such unique structure provides the CNTs with exceptional electrical and thermal conductivity, high strength and stiffness and enormous aspect ratio. These properties enable the development of electrically conductive polymeric composites with very low CNT loading and can provide improved mechanical performance to a polymeric matrix for applications ranging from electronic to aerospace and automotive industry. Potential applications include conductive structure materials for aerospace or automotive industry, electromagnetic interface (EMI) shielding materials, dissipative materials and thermal management materials for the microelectronic industry and potential transparent field emission materials for display and other electronic applications.
According to percolation theory, a three dimensional CNT conductive network in polymer matrix is needed to provide a conductive path. The percolation threshold is characterized by a sharp increase in conductivity coinciding with the formation of a three dimensional conductive network. Thus, a key factor to achieve reasonable conductivity is the proper dispersion of a CNT filler in a polymeric matrix. In past years several techniques have been developed to efficiently disperse CNTs in a polymeric matrix. The commonest method is direct mixing of CNTs and polymer through melt blending or shear-intensive mechanical stirring (Moisala A, Li Q, Kinloch I A, Windle A H, Thermal and electrical conductivity of single and multi walled carbon nanotube epoxy composites, Composites science and technology, 2006, 66, 1285; Li Z F, Luo G H, Wei F and Huang Y, Microstructure of carbon nanotubes/PET conductive composites fibers and their properties, Composites science and technology, 2006, 66, 1002; Sandler J K W, Kirk J E, Kinloch I A, Shaffer, M S P and Windle A H, Ultra low electrical percolation threshold in carbon nanotube epoxy composites, Polymer, 2003, 44, 5893; Lavin J G and Samuelson H V, Single wall carbon nanotube polymer composites, U.S. Pat. No. 6,426,134). However this method is generally not very effective at dispersing CNTs in polymers and is limited to thermoplastics or low viscosity polymers.
In another dispersion method, a solvent was employed to lower the viscosity of polymer and facilitate the dispersion of CNTs. With this method, CNTs are first exfoliated into an organic solvent under high-power ultrasonication. Then the CNT suspension is mixed with polymer, and the organic solvent is allowed to evaporate (Kim Y J, Shin T S, Choi H D, Kwon J H, Chung Y C and Yoon H G, Electrical conductivity of chemically modified multiwalled carbon nanotube/epoxy composites, Carbon, 2005, 43, 23; Li N, Huang Y, Du F, He X B and Eklund P C, Electromagnetic interference shielding of single walled carbon nanotube epoxy composites, Nano Lett, 2006, 6, 1141; Connell J W, Smith J G, Harrison J S, Park C, Watson K A, Ounaies Z, Electrically conductive, optically transparent polymer/carbon nanotube composites and process for preparation thereof, U.S. Pat. 2003/0158323). Compared to the earlier described method, the dispersion of CNTs in the polymer using this method is better. However high-power ultrasonication for a long period of time generally shortens the nanotube length and destroys its integrity, which is detrimental to the conductivity of the resulting composite. Also, during slow solvent evaporation, nanotubes tend to agglomerate, leading to inhomogeneous distribution in the polymer matrix. Another problem with the solution blending method is the use of toxic and flammable solvents.
Grunlan et al describe an approach to incorporating CNTs into a polymeric matrix with relatively low percolation threshold based on the use of latex technology (Grunlan J C, Mehrabi A R, Bannon M V, Bahr J L, Water based single walled nanotube filled polymer composite with an exceptionally low percolation threshold, Adv Mater 2004, 16, 150). Initially, CNTs and polymer particles were uniformly suspended in a solvent. Once most of the solvent had evaporated, the polymer particles assumed a close-packed configuration with CNTs occupying interstitial space. Finally, the polymer particles were coalescenced together to form a coherent film locking the CNTs within a segregated three dimensional network. In this processing method solid polymer particles created excluded volume to reduce the free volume available for the CNTs to form a conductive network. As a result, the percolation threshold was significantly reduced.
The interfacial interaction between CNTs and a polymeric matrix will affect the compatibility of CNTs with the matrix, and hence their dispersion in the matrix. Thus, both modification of the CNTs by functionalization of their walls and modification of the polymer matrix have been employed to promote the dispersion of CNTs.
While the prior dispersion techniques may be generally satisfactory for their respective systems, these techniques are quite limited in their ability to fabricate conductive thin films with controlled thickness, especially on irregular-shaped substrates.
Electrophoretic deposition (EPD) is a widely used industrial colloidal process to produce thin films on conductive substrates. In EPD, charged particles suspended in a liquid medium are attracted and deposited onto an oppositely charged conductive electrode in a DC electric field (Berra L and Liu M L, Progress in materials science, 2007, 52, 1). EPD has advantages of short film formation time, simple apparatus, continuous fabrication, good homogeneity and packing density and suitability for mass production as in electric coating industry. Most importantly, it can be used to fabricate thin film onto variously-shaped surfaces with controlled thickness and morphology. Patterned deposition can also be achieved by using masked electrode. EPD has been used to produce pre-fabricated CNTs (Boccaccini A R, Cho J, Roether J A, Thomas B J C, Minay E J and Shaffer M S P, Electrophoretic deposition of carbon nanotubes, Carbon, 2006, 44, 3149), and such fabricated CNTs films show good electron field emission stability under both continuous and pulsed operations (Gao B, Yue G Z, Qiu Q, Cheng Y, Shimoda H, Fleming L and Zhou O, Fabrication and electron field emission properties of carbon nanotube films by electrophoretic deposition, Adv Mater 2001, 13, 1770).
Polyimides (PI) are excellent in heat resistance, chemical resistance and mechanical properties. They are widely used in aerospace and automotive industry and also play important role as dielectric layers in a variety of microelectronic devices. In microelectronics industry, PI films are commonly produced by film casting of a non-aqueous polyamic acid precursor solution followed by heat curing. The various casting methods include air spraying, roll coating, brush coating and dip coating. However, irregular-shaped objects cannot be easily provided uniform coating films by these methods.
To solve this problem, EPD has been employed, and has shown some additional advantages such as small loss in coating materials and uniform thin film with controlled thickness. A continuous coating of PI onto electrical conductor has been disclosed in U.S. Pat. No. 3,846,269 (Marcello N E, Creek T and Phillips D C, Method for continuous coating of polyimide by electrodeposition). In this method a coated electrical conductor is made by continuously passing a positively charged electrical conductor near a negatively charged electrode in a bath of a conducting non-aqueous polyamic acid suspension. A photosensitive polyimide having oxycarbonyl groups in side chains has been developed and employed to fabricate a patterned PI film through EPD followed by photolithography (Hiroshi I and Shunichi M, Composition for polyimide electrodeposition and method of forming patterned polyimide film with the same, EP 1 123 954).
In the microelectronics industry, the adhesion strength between PI and metallic substrate is a crucial factor influencing the performance of electronic devices. PI is known to adhere poorly to metals, especially to copper, and is easily delaminated from a copper substrate. It was found that acid groups of polyamic acid can react with copper to produce copper ions. These copper ions can diffuse into the PI layer to accelerate the oxidation of PI during heat curing at elevated temperature (Chamber S A, Loebs V A and Chakravorty K K, Oxidation of Cu in contact with preimidized polyimide J Vac Sci Technol 1990, A8, 875). To prevent the diffusion of copper ion and maintain the adhesion strength of the PI/copper interface, a barrier film such as Cr, Ni or Ta is always inserted between PI and copper (Ghosh M K and Mittal K L, Polyimides: fundamentals and applications, New York: Marcel Dekker, 1996). However, this method is not simple or cost effective. Polyvinylimidazole (PVI) and its silane derivatives have been developed to prevent corrosion of PI layer at high temperature (Jang J and Earmme T, Interfacial study of polyimide/copper system using silane modified polyvinylimidazoles as adhesion promoters, Polymer, 2001, 42, 2871). These materials suppress the corrosion of copper and the diffusion of copper ion into PI through complex formation with copper (Xue G, Shi, G, Ding J, Chang W, Chen R, Complex-induced coupling effect-adhesion of some polymers to copper metal promoted by benzimidazole, J Adhesion Sci Technol, 1990, 4, 723). On the other hand, silanes are an effective adhesion promoter of PI/inorganic interface (Linde H G and Gleason R T, Thermal stability of the silica-aminopropylsilane-polyimide interface, J Polym Sci Chem Ed 1984, 22, 3043). However, the application of PVI and its silane derivatives requires casting these primers onto the copper substrate before application of the PI, which makes the processing more complicated.
Thus, there is a need for an efficient method to produce CNT-filled composite thin films through EPD with tunable thickness and electrical conductivity. There is also a need for a method to increase the adhesion strength of a polymeric thin film to a metallic substrate through incorporating CNTs into the film.